The present invention relates to optical systems for viewing a display, and more particularly, to an optical system which utilizes a folded optical path to minimize the distance between the display and the viewer.
To simplify the following discussion, the present invention will be discussed in terms of displays utilized in head mounted computer displays; however, it will be apparent to those skilled in the art from the following discussion that the present invention may be applied to other types of imaging systems. Head-mounted computer displays may be viewed as xe2x80x9ceye glassesxe2x80x9d that are worn by the user to view images created by a computer or other image source. The image seen by each eye is generated on a display screen having a two dimensional array of pixels.
It is advantageous to minimize the distance between the display and the eye of the viewer to minimize the portion of the display that extends from the viewer""s face. Large overhanging displays are uncomfortable to wear. In addition, it is advantageous to be able so see around the display, so the folded optical system and display should not occlude the user""s vision of the surrounding view.
Prior art systems that combine reflective optics with short display to eye distances are known to the art. These systems typically utilize partially reflecting optical surfaces to fold the optical path so that the distance from the viewer""s eye to the display is minimized. Such a system is described in U.S. Pat. No. 5,644,436. For such systems to operate, the direct light emitted by the display must be blocked. Prior art systems typically utilize polarization filters and quarter wave plates to block the direct light from reaching the eye. Unfortunately, quarter wave plates only rotate the polarization vector of the light through precisely 90 degrees for specific wavelengths. Light having wavelengths that differ from the design wavelength is rotated either through slightly more than 90 degrees or slightly less than 90 degrees. Accordingly, these prior art systems do not block all of the light coming directly from the display and the viewer sees xe2x80x9cghostxe2x80x9d images.
In addition, the optical system taught in the ""436 patent is not telecentric. A telecentric optical system is one in which the chief rays are all perpendicular to the display. Since the display is reflective, a non-telecentric display must vary the angle of incidence of the illuminating light with position on the display to provide an image, which is uniform in intensity. If liquid crystal displays are utilized, however, the presence of non-normally incident light on the display makes it difficult to optimize the liquid crystal display elements.
Finally, the optical system taught in the ""436 patent is inefficient in its use of light. Less than {fraction (1/16)}th of the light leaving the light source actually reaches the observer""s eye. Accordingly, a much larger light source is needed. The larger light source increases the size of the display and the power needed to run the display.
Broadly, it is the object of the present invention to provide an improved folded optical system based on reflective optical imaging elements.
It is a further object of the present invention to provide an optical system in which light from the display being imaged is blocked from reaching the viewer""s eye independent of the wavelength of the light.
It is yet another object of the present invention to provide an optical system that is substantially telecentric.
It is a still further object of the present invention to provide an optical system that requires a lower intensity light source Man prior art systems.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an optical imaging assembly for imaging light from a display. The optical assembly is constructed from first and second linear polarization filters, first and second lenses that are preferably convex-concave lenses, and first and second xc2xc wave plates. The first linear polarization filter passes light polarized in a first direction, and the second linear polarization filter passes light polarized in a second direction that is orthogonal to the first direction. A folded imaging assembly is constructed from the first and second lenses and the first xc2xc wave plate. The first and second lenses have partially reflecting coatings on one surface of each lens. The folded imaging assembly and the second xc2xc wave plate are located between the first and second linear polarization filters. In the preferred embodiment of the present invention, the first xc2xc wave plate has a birefringence axis that is orthogonal to the birefringence axis of the second xc2xc wave plate and the two xc2xc wave plates are constructed from the same material. The partially reflective coating of one of the first and second lenses preferably includes a material having a reflectivity that depends on the direction of linear polarization of light striking the reflective coating. The surfaces of the convex-concave lenses are preferably spherical. The lenses preferably provide a substantially telecentric optical imaging system.